1. Field of the Invention
The present invention relates to a method of driving a door of an automatic door assembly and more particularly, to a method of driving the door by a linear motor mounted within the automatic door assembly.
2. Description of the Prior Art
Automatic door assemblies having a linear motor as a prime mover are advantageous in that the linear motor can drive the door linearly without any special driving power transmission mechanism, the motor is simple and durable in construction, and it can be manufactured less costly. However, they also have disadvantages such that when the propulsion force of the linear motor is too small, the door is retarded and tends to stop before it reaches the end of its stroke, and when the propulsion force is too great, the door is driven so rapidly that the door frame is subjected to the full impact of the moving door. Various attempts have heretofore been made to stop the door exactly at the ends of the door stroke by retarding the door during its stroke, thereby preventing the door from striking the outer frame. One such attempt has been to reduce the speed of the door electrically by giving an opposite propulsion force to the linear motor during a final portion of the door stroke. However, this attempt has led to a drawback in that various intricate control devices such as a speed detecting device and a position detecting device which must be adjusted precisely need to be added to the automatic door assembly. Another such attempt has been to provide a pair of cushioning devices such as self-returning type air cylinders at the ends of stroke of the movable door so as to dampen the door speed mechanically.
A problem with the automatic door assembly having the cushioning devices is that the propulsion force of the linear motor must be held at all times to a level sufficiently large to overcome frictional resistance of the door and linear motor and reaction force of the cushioning device. Thus, when the linear motor, especially its reaction rod, is subjected to a voltage decrease (arising from fluctuation of a power supply) and a temperature rise (due primarily to frequent opening and closing of the door), the propulsion force of the linear motor is decreased, and the door tends to be stopped by the reaction force of the cushioning device before reaching the end of the door stroke. This result is disadvantageous especially when the door is to be closed.
One solution to the above shortcoming would be to provide a feedback control device whereby the voltage fluctuation of the power supply and the temperature rise of the linear motor are detected to automatically correct the reduction of the propulsion force of the motor. This solution is however also disadvantageous because the overall structure of the automatic door assembly becomes much more complicated.